The invention relates to an improved use of servo track information to follow a track during a read operation. The track is situated on a rotating disk surface in a hard disk drive. Hard disk drives include at least one rotating disk surface accessed by a read-write head mechanically coupled to an actuator arm in a voice coil motor. The invention involves improving the control of at least the voice coil motor, and possibly a micro-actuator. The micro-actuator may be used to provide refinement of the positioning of the read-write head.
The voice coil motor is controlled through electrical stimulation of its voice coil, which interacts with the fixed magnet to pivot at least one actuator arm through the actuator pivot. As the actuator arm pivots, the read-write head is positioned over a data track on the rotating disk surface.
Tracks on a rotating disk surface include both a data track and a servo track. Servo tracks provide positioning information written onto the rotating disk surface. Typical servo track information includes a gray code representation of the track, as well as positional correction information. While in the past a separate rotating disk surface was sometimes reserved for the servo track information, today it is common for the servo track information to be multiplexed with the data on each rotating disk surface to be accessed. The data track is where the data for the application system is stored by the hard disk drive.
Different methods are used to position the read-write head for reading and for writing the data track. When reading the data track, the read head is positioned to follow the servo track of the data track. When writing the data track, the read head is positioned near a different servo track located some distance from the data track. This distance is the distance between the read head and the write head, which today is often over twenty tracks apart.
The process of writing the servo tracks onto a rotating disk surface is known as servo writing. Servo writing may be done inside an assembled hard disk drive. Alternatively, servo writing may be performed before the disks are assembled in the hard disk drive. Given the reliability of the disks today, there are economic advantages to assembling the hard disk drive before servo writing.
Each track on a rotating disk surface typically conforms to an overall structure. For example, a track often includes multiple sectors. Each sector typically includes a collection of at least two, often four and sometimes six Position Error Signal (PES) bursts. While it is possible for an odd number of PES burst signals to be useful, the discussion herein will focus on even numbers of PES burst signals. These PES bursts are written as part of the servo write process. The servo write process is used to operationally define the tracks on the rotating disk surface. The tracks, once operationally defined, persist when the power is turned off to the hard disk drive. The PES bursts allow the servo controller of the hard disk drive to sense the position of the read head over a track to a fraction of the track width. The fraction of the track width may be one half or less of the track width.
Mechanical vibrations are often experienced during the servo writing of tracks. These vibrations may result from external and/or internal vibrations. Mechanical vibration during the servo writing of tracks may result in the PES bursts for a track following a trajectory not exactly matching the track center. The consequence of this trajectory discrepancy is that the PES burst information may mislead a servo-controller, potentially degrading the ability of the hard disk drive system as a whole to position a read head to follow the track.
To minimize the possibility of PES burst trajectory discrepancies misleading the servo controller, several attempts to correct this problem are found in the prior art. Most of these attempts are algorithms designed to correct the trajectory discrepancies found in the PES bursts for a track. Most are based upon some form of iterative learning process. These iterative learning processes tend to collect PES values derived from calculations based upon sampling the track for multiple disk rotations. These collected PES values and/or results of the calculations are usually written to the disk for the tracks showing trajectory discrepancies.
These prior art corrective measures tend to add to the production cost by adding to the time required to initialize the rotating disk surfaces within assembled hard disk drives. In order to minimize production cost, data tracks are scanned to determine the quality of their PES bursts in matching the track trajectory. The prior art PES corrective algorithms are only applied to those tracks with the worst PES bursts. Data tracks with very bad PES quality for write mode are often rejected, rather than incur added production costs.
It is common in the prior art for the metric defining PES quality to differ between read mode and write mode. The PES quality for write mode is commonly seen as more important than the PES quality for read mode. Consequently, the acceptable PES quality for read mode tends to be lower than for write mode.
These conditions in the prior art lead to the following situation. When a hard disk drive has one or more read errors for a track due to poor PES quality for write mode, there is a common approach taken to correct this situation. The approach adjusts the positioning offset around the track for the read head. The read head then attempts to access the track. Depending on the PES quality for write mode, it may take several offset attempts, each for at least one disk rotation, to successfully access the track. This approach is known as an off-track then read retry sequence.
The off-track then read retry sequence is time consuming, and often adds to production expenses during quality testing of assembled hard disk drives. Consequently, it is a common production practice to turn off the off-track then read retry sequence during the initialization of rotating disk surfaces within assembled hard disk drives. This can lead to tracks with poor PES quality for write mode causing their hard disk drives to fail production testing.
What is needed is a quick way to offset the read head position around a track with a high probability of success, when the track has failed to be read. This need extends both to the initialization of a hard disk drive after assembly, as well as to the hard disk drive in normal operation.